Human growth hormone, as isolated from the pituitary gland, has been found to be a family of different forms. All seem to be enzymic modifications. Even preparations used in radioimmune and radioreceptor assays as "pure" reference material are contaminated with the various forms. In addition to isolating these forms from pituitary extracts, we deliberately modify the hormone by enzymic means and determine effects on the biological activity. We are pursuing the idea that growth hormone must be modified for full biological activity.